Heavy medium beneficiating process

ABSTRACT

A process for heavy medium beneficiating of coal, including particles in the 28 X 150 mesh range. As-mined or crushed coal is washed and screened to prepare a feed material whose average coarseness is greater than about 150 mesh. This feed is combined with a heavy medium, such as water and magnetite, and cycloned. Cyclone overflow is screened, drained and rinsed to separate returnable heavy medium, fines (both heavy medium and ore) from coarse product. Fines are processed through a magnetic separator to separate the heavy medium from a non-magnetic fines ore-water fraction. The ore-water fraction is screened to separate a fine product and recirculatable water. Cyclone underflow is similarly screened to separate returnable medium, fines (medium, gangue and ore particles) from coarse gangue. The fines are concentrated in a densifier and then magnetically separated into a magnetic and non-magnetic fraction. The magnetic fraction is reused as heavy medium and the non-magnetic fraction is screened to recover recirculatable water and a particle fraction which requires further processing.

United States Patent n91 Miller et a1.

[ Feb. 26, 1974 HEAVY MEDIUM BENEFICIATING PROCESS Primary Examiner--Frank W. Lutter Assistant ExaminerRa1ph J. Hill 5 7 ABSTRACT A process for heavy medium beneficiating of coal, including particles in the 28 X 150 mesh range. Asmined or crushed coal is washed and screened to prepare a feed material whose average coarseness is greater than about 150 mesh. This feed is combined with a heavy medium, such as water and magnetite, and cycloned. Cyclone overflow is screened, drained and rinsed to separate returnableheavy medium, fines (both heavy medium and ore) from coarse product. Fines are processed through a magnetic separator to separate the heavy medium from a non-magnetic fines ore-water fraction. The ore-water fraction is screened to separate a fine product and recirculatable water. Cyclone underflow is similarly screened to separate returnable medium, fines (medium, gangue and ore particles) from coarse gangue. The fines are concentrated in a densifier and then magnetically separated into a magnetic and non-magnetic fraction. The magnetic fraction is reused as heavy medium and the nonmagnetic fraction is screened to recover recirculatable water and a particle fraction which requires furtherv processing.

8 Claims, 2 Drawing Figures REC/11 6024760 14/4751? [75] Inventors: Francis G. Miller, Bethlehem;

Stanton D. Irons, Northampton, both of Pa.

[73] Assignee: Bethlehem Steel Corporation,

Bethlehem, Pa.

[22] Filed: Mar. 6, 1972 [21] Appl. No.: 231,852

[52] U.S. C1. 209/172.5

[51] Int. Cl B03b 3/44 [58] Field of Search 209/12, 39, 172.5

[56] References Cited UNITED STATES PATENTS 2,387,866 10/1945 Walker 209/172.5

3,023,893 3/1962 Zaborowski 209/18 2,889,925 6/1959 Fontein 209/172.5

2,984,355 5/1961 Leeman 209/172.5

3,446,349 5/1969 Benzon 209/17 3,638,791 2/1972 Harrison 209/12 FOREIGN PATENTS OR APPLICATIONS 132,480 5/1949 Australia 209/39 /7- REC/RCUL 4750 Mrs: FfED Ric/RC0: Amara /5 M; I'ER 70 FINES PRDCESS/A/G CIRCUIT PATENTED 3,794,162

' sum 20; 2

1 HEAVY MEDIUM BENEFICI'ATING PROCESS BACKGROUND OF THE INVENTION This invention relates to classifying, separating and assorting solids, and more particularly to gravity liquid floating processes.

Specific gravity processes for beneficiating mineral particles, such as coal, generally 'fall'into two categories. The first category includes processes which employ water only as the separating'medium, and thesecond category includes processes which rely on a medium heavier than water, e.g.-adense orheavy medium.

The density at which separation is most desirable is the density closest to that of the desired product, thus enabling the product to be floated away from the gangue. Coal has an average density of 1.3, while the density of the gangue associatedwith the coal is-greater than 1.3. The purpose of using heavy medium, as opposed to water (whose density is 1.0) alone, derives from these density differences.

Thus, when water alone is used as a coal separating medium, it is necessary to employ hydrodynamic conditions that increase apparent density of the water. However, a dense medium has the advantage in that the specific gravity is easily manipulated to an exact predetermined point between that of all coal and all gangue. This in turn permits a more efficient separation. Efficient separation is necessary in coal processing to economically provide a desired quality of product.

The following tabulation of the operating characteristics of typical coal-water and coal-heavy mediumseparating units is presented for comparison.

Lower robable error is more efficient.

In Table 1, column 1 indicates the unit used to effect the separation. Column 2 gives the unit efficiency as measured by probable error. Probable error and efficiency are well known concepts in the art and detailed discussions may be found, for example, in Coal Preparation, .I.W. Leonard, and D. R. Mitchell, 1968, American Institute of Mining, Metallurgy and Petroleum Engineers, lnc. Column 3 indicates a typical size of the coal processed, and column 4 indicates the lowest density at which the process may be practically operated for the typical size noted.

Table 1 demonstrates that (a) a separating process using heavy medium is the most efficient unit for beneficiating coal and also (b) one that will maintain high efficiency at low densities.

However, even though heavy medium separation is known to have the advantages of high efficiency at low density, industrial processes utilizing heavy medium for very fine, e.g. smaller than about 28 mesh, coal are unknown. When heavy medium is employed in conventionalprocesses for beneficiating particles in the 28 to 150 mesh size, the following problems occur:

1. The buildup of recirculating, fine, non-magnetic solids (finer than 28 mesh) in the heavy medium impairs separation efficiency and overall circuit performance, resulting in (a) loss of coal to refuse, (b) contamination of coal product with refuse, and (c) increased heavy medium losses. It has been estimated, for example, that 28 X mesh particles contamination buildup in the heavy medium averages about tons perhour for a 320 ton per hour circuit.

2. The water clarification circuit and the load on the recirculating water system are excessive and costly due to the extensive purging of fine non-magnetic solids from the medium and the attendant inability to recirculate water before clarification.

3. Extensive and costly raw coal screening systems for sizing at 100 or 150 mesh are required to cope with the present inefficient fine screening systems to prevent material finer than 150 mesh from entering the heavy medium system.

4. Extensive and costly fine screening systems are required to recover the heavy medium from the product and gangue.

Current and proposed stringent pollution control dictates use of low sulfur coal. With the depletion of much of the best low sulfur coal deposits, lower quality reserves must be upgraded to meet the strict specifications of lower sulfur content required by current and proposed regulations on pollution control. Since these low quality reserves contain more bound fine impurities, i.e., fine sulfur constituents, it is necessary to crush the coal to finer sizes to properly beneficiate the coal.

Therefore, there has been a need for a mineral beneficiating process which will l efficiently and economically beneficiate ore, including 28 X 150 or finer mesh, with a heavymedium, and (2) provide an economically feasible way to re-use and recover the medium.

SUMMARY OF THE lNVENTlON We have discovered a process for separating minerals, including fine particles between about 28 and about 150 mesh which obviates the aforementioned difficulties. In the process of this invention crushed, or as-mined minerals such as coal, are first screened to remove particles below about 150 mesh. Following initial screening and washing, the coal is combined with a heavy medium, such as a water slurry of magnetite ore, and cycloned, or similarly processed, to produce an overflow and underflow fraction. The overflow fraction is screened, with heavy medium returned to the sump, drained to separate fines and more heavy medium and, finally, rinsed with water to yield a coarse product (essentially plus 28 mesh), and a drained fine fraction consisting of fine non-magnetic solids and magnetite. The latter is then processed in a magnetic separator to yield a fine non-magnetic fraction and a heavy medium. The heavy medium is returned to the system and the nonmagnetic fraction is screened to yield a fine coal product (essentially 28 X 150 mesh), and wash water which is returned to the system.

The cyclone underflow is also similarly screened,

drained and washed to yield a coarse gangue, which is discarded and a fine (coal, gangue and magnetite) fraction which is concentrated in a densifier or thickener. Densifier overflow provides wash water for the system and densifier residue, i.e., sludge, is processed by a magnetic separator to produce a heavy medium which is returned to the system, and a combined fine coal and gangue particle fraction which is screened to allow the tailings water to be recirculated.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a schematic representation of an embodiment of this invention.

FIG. 2 shows an alternate embodiment by which the process of this invention may be practiced.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a feed of crushed or raw coal is screened and washed on screen to remove essentially l 50 mesh fines. The coarse solids (in a size fraction as coarse as l /zinches and as fine as ISO mesh) enter sump 14 by line 13 where they are combined with a heavy medium composed of magnetite ore and water to form a slurry. The slurry is pumped by pump 16 through line 17 to cyclone 18 where an overflow 20 and underflow 22 are produced.

The overflow 20 is screened on sieve bend 24, drained on drain 26 and rinsed and drained on drain 28 to produce a coarse product and a fine medium. A portion, or all, of the fine medium from sieve bend 24 is returned to sump 14 by line 25 and another portion is fed to line 34 by line 35. Rinse water is supplied to drain screen 28 by line 30 from densifier 32 and/or magnetic separator tailings water from lines 49 and 61.

The fine medium and fine product leave drain 26 by line 34 and are diluted as required with a portion of drain water from line 39. The combined streams of lines 34, 35, 39 converge into line 100 and go to magnetic separator 36, and from there the magnetic fraction is returned by line 37 to sump 14. The nonmagnetic fraction produced inamagnetic separator 36 passes to sieve bend 48 by line 46, where a fine product and wash water 49 is produced. Wash water from drain 28 enters sump 40 by lines 38 and 72, and is pumped by pump 42 to densifier 32 by line 44. Underflow from cyclone 18 leaves by line 22 to sieve bend 62 where a coarse gangue and a fine fraction are separated. A portion of the fine fraction from sieve bend 62 is returned to sump 14 by line 65 and another portion is sent to line 72 by line 70. The coarse gangue from sieve bend 62 is drained on drain 64 and rinsed and drained on drain 66 with wash water received from densifier 32 by line 68 and/or magnetic separator tailings water from lines 49 and 6l. Drain water from drains 64 and 66 pass to sump 40 by lines 70 and 72, and from there through pump 42 to densifier 32. Alternately, drain water at 72 may be sent directly to magnetic separator 52 if densifying is deemed unnecessary. Thickener sludge leaves densifier 32 by line 50 to magnetic separator 52 where it is converted to a magnetic fraction which enters sump 56 by line 54 and from there returns to sump 14 by line 58. When required, new heavy medium may be added to the circuit at 56. The non-magnetic fraction from magnetic separator 52 travels by line 53 to sieve bend 60 where wash water and a material requiring further treatment are produced. Wash water produced from sieve bends 48 and is recycled to the circuit at various places where needed, such as, for example, drains 10, 28 or 66.

An alternate embodiment shown in FIG. 2 provides for an initial rough cut to be made at sieve bend 24 and 62 followed by another screening at 24' and 62, respectively. Beginning at cyclone 18, overflow proceeds to sieve bend 24 by line 20 where it is screened on sieve bend 24, drained on drain 26, and rinsed and drained on drain 28 as in FIG. 1. In the embodiment of FIG. 2, the mesh size of sieve bend 24 and drain 26 are the same, or nearly the same. Fines from sieve bend 24 and drain 26 flow by line 25 and 34' to sieve bend 24 for further screening. Fine medium and fine product from sieve bend 24' enter magnetic separator 36 by line 100 where further separation is performed as in FIG. 1. Line 25' returns medium from sieve bend 24' to sump l4.

Underflow from cyclone 18 is likewise screened twice, moving from cyclone 18 by line 22, to sieve bend 62. Sieve bend 62 is of the same or nearly the same mesh as drain 64.

Underflow from sieve bend 62 and drain 64 pass to sieve bend 62 by lines 65 and Drainage from sieve bend 62' returns to sump 14 by line 65 and a fines fraction enters sump 40 by line 71.

In the practice of this invention it has been found that screening can be satisfactorily accomplished by use of a sieve bend of the type described in U. S. Pat. No. 2,916,142 issued Dec. 8, 1959 and U. S. Pat. No. 3,466,349, issued May 27, 1969. The screen size may be varied so that, for example, sieve bends Nos. 24, 48, 60, 62 may be designed to screen at 28 mesh, 60 mesh, etc., depending on the crushed ore size. Also, a sieve bend may be substituted for screen 10. However, it is preferable for the process of this invention that drains 26 and 28 allow passage of a coarser average material than that which passes through sieve bend 24. Similarly drains 64 and 66 can be selected or constructed so that a coarser average material will pass through them than passes through sieve bend 62. A preferred mesh for coal beneficiating in FIG. 1 would be about to mesh at sieve bends 24 and 62, and about 28 mesh for drains 26, 28, 64 and 66. However, in FIG. 2, sieve bends 24 and 62 may be 28 mesh while sieve bends 24 and 62 would be 150 mesh, with the drain mesh in the latter remaining at about 28 mesh. Wash water herein refers to both fresh water and recirculated water as indicated.

Use of sieve bends, 48 and 60, are preferred in this process; however other screens, cyclones as well as chemical froth floation may be used.

The specific gravity of the heavy medium applicable to our process may be varied to correspond to analysis of the ore product, as is well known in the art.

Heavy medium herein refers to an aqueous suspension of particles, such as magnetite or other solutions or suspensions, as is well known in the art.

Densitier 32 may be a thickener, cyclone or other device which densifies or thickens suspended particles.

Although the primary application of this invention is with 28 X 150, or finer mesh, coal, the subject heavy medium process is applicable also to present coarse coal processing systems which treat material as coarse as 3". Improvements to both the aforementioned fine and coarse systems, are, for example:

l. Magnetite losses are reduced by recirculating magnetic separator tailings as wash water to the rinse portion of the vibrating screen.

2. Overall circuit performance is improved by reducing the amount of non-magnetics in circulation, since the process of this invention continually purges the system of fine solids that can build up from ineffieient raw coal screening and/or degradation.

3. By eliminating the bulk of the recirculating fine solids from the medium, the process of this invention allows the feed into a heavy medium system to be in creased and still maintain a proper coal to medium weight ratio of about 1 to 5. Thus, processing costs per ton treated are reduced.

4. The process of this invention permits circulations of cleaner medium which effects more perfect (sharper) separations, thereby improving product quality and thus reducing the loss of coal to refuse.

5. Water clarification costs are reduced, since water exiting heavy medium circuits for clarification is now clean enough to be recycled.

6. Fine coal product that now recirculates and must be processed by other means (fine coal tabling, hydrocycloning, etc.) can be recovered directly, thus reducing fine coal processing costs.

7. Savings can be realized by reducing the size of the raw coal screening system since the process ofthis in vention copes with a greater proportion of fines entering with the raw feed which in turn allows less efficient screening systems to be used.

8. Raw coal screening will not have to cope with the fines exiting from the heavy medium system via the magnetic separator tailings, as is now conventional.

In addition to the above benefits, the following advantages of this invention are particularly applicable to fine, i.c. finer than 28 mesh, coal processing.

1. Separations as low as L30 specific gravity are possible, as compared to present fine coal systems which cannot efficiently separate below 1.50 specific gravity.

2. Coal as fine as 150 mesh may be processed more efficiently than is now possible with hydrocyclones, tables or other fine coal processing units.

It should be understood that while the process of this invention is especially applicable to coal, other ores, for example, fluorspar and potash may be substituted.

We claim:

1. In a process of ore separation by heavy medium where the ore includes particles finer than 28 mesh, and in which the ore and heavy medium are separated into an overflow float ore fraction and an underflow sink ore fraction, and which process includes:

a. a first screening separation of the overflow fraction to separate heavy medium from a float ore fraction;

b. draining of the float ore fraction of step (a) to separate a coarse ore fraction from a fines fraction, the latter including heavy medium and ore;

c. magnetically separating the fines fraction from step (b) into heavy medium and a float fines orewater fraction;

d. a first screening of the underflow sink fraction to separate heavy medium from a sink ore fraction;

e. draining of the sink ore fraction of step (d) to separate a coarse ore fraction from a fines sink fraction,

the latter including heavy medium, and magnetically separating the fines fraction from step (e) into heavy medium and a fines sink fraction;

the improvement comprising:

1. restricting the screening at step (a) and step (d) to no coarser than about 28 mesh;

2. returning a portion of the drainage from (b) and (e) to the heavy medium source without further particle separating treatment;

3. restricting the magnetic separation at (c) and (f) to single stage separation;

4. screening the fines float fraction of step (c) to produce a fines particle fraction and recirculatable water;

5. screening the fines sink fraction of step (f) to produce a sink particle fraction and recirculatable water.

2. The process of claim 1 in which the ore is coal.

3. The process of claim 1 in which, following step (a) there is a second screening separation at a finer mesh size than the first screening separation.

4. The process of claim 3 in which the second screening separation is at about lOO mesh size.

5. The process according to claim 3 in which the first and second screening are of the sieve bend type and the drainage screens are of the mesh type and in which the drainage screening is at the same effective mesh as the first screening and all the throughput from the first screening and draining are processed by the second screening.

6. The process of claim 1 in which the following step (e) there is a second screening separation at a finer mesh size than the first screening separation.

7. The process of claim 6 in which the the second screening separation is at about mesh size.

8. The process according to claim 6 in which the first and second screening are of the sieve bend type and the drainage screens are of the mesh type and in which the drainage screening is at the same effective mesh as the first screening and all the throughput from the first screening and draining are processed by the second screening. 

2. returning a portion of the drainage from (b) and (e) to the heavy medium source without further particle separating treatment;
 2. The process of claim 1 in which the ore is coal.
 3. The process of claim 1 in which, following step (a) there is a second screening separation at a finer mesh size than the first screening separation.
 3. restricting the magnetic separation at (c) and (f) to single stage separation;
 4. screening the fines float fraction of step (c) to produce a fines particle fraction and recirculatable water;
 4. The process of claim 3 in which the second screening separation is at about 100 mesh size.
 5. The process according to claim 3 in which the first and second screening are of the sieve bend type and the drainage screens are of the mesh type and in which the drainage screening is at the same effective mesh as the first screening and all the throughput from the first screening and draining are processed by the second screening.
 5. screening the fines sink fraction of step (f) to produce a sink particle fraction and recirculatable water.
 6. The process of claim 1 in which the following step (e) there is a second screening separation at a finer mesh size than the first screening separation.
 7. The process of claim 6 in which the the second screening separation is at about 100 mesh size.
 8. The process according to claim 6 in whIch the first and second screening are of the sieve bend type and the drainage screens are of the mesh type and in which the drainage screening is at the same effective mesh as the first screening and all the throughput from the first screening and draining are processed by the second screening. 